J. Phys. Chem. B 2003, 107, 2887-2891
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ARTICLES An Electron Energy-Loss Spectrometry Study of Charge Compensation in LiNi0.8Co0.2O2 J. Graetz,* C. C. Ahn, R. Yazami, and B. Fultz DiVision of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125 ReceiVed: July 9, 2002; In Final Form: January 6, 2003
Changes in the electronic structure of Li1-xNi0.8Co0.2O2 during delithiation (charge) are elucidated by changes in the O K-edge and transition-metal (TM) L2,3-edge. Electron energy-loss spectrometry was used to investigate the O 2p and TM 3d occupancy over a wide range of lithium concentrations. Quantitative analysis of the TM L2,3 white lines indicates that the Ni ion accommodates no more than one-half of an electron, whereas the net Co valence is unaffected by delithiation. In contrast with the small changes observed in the TM L2,3-edge, a large decrease in state occupancy is observed in the near-edge structure of the O K-edge, suggesting that much of the electron charge is accommodated by the anion.
1. Introduction Lithium transition-metal oxides, LiTMO2, are in widespread use as cathodes in secondary lithium batteries. These high-voltage cathodes maintain stable electrochemical behavior upon lithium deintercalation and exhibit excellent cycle life. LiNi1-yCoyO2 (0 < y < 1) has a layered structure with rhombohedral symmetry and belongs to the R3hm space group. The compound LiNi0.8Co0.2O2 is a promising cathode for lithium batteries. It has the same layered structure as LiCoO2 but with a substitution of Ni atoms at the Co sites. In LiNi0.8Co0.2O2, the Ni and Co ions are both octahedrally coordinated by O ions and occupy the same crystallographic sites. This material is an improvement over LiNiO2, because it combines a high cycling capacity with safe operating conditions even in the delithiated charge state. LiNi0.8Co0.2O2 remains as a single phase during deep lithium deintercalation, allowing reversible cycling from Li1.0 to Li0.4.1 Also, LiNi0.8Co0.2O2 is less susceptible to cationic disorder (TM ions occupying Li sites) than the LiNiO2 system. LiNi0.8Co0.2O2 has a slightly lower voltage versus lithium, when compared to LiCoO2; consequently, electrochemical cells are less prone to electrolyte decomposition. Finally, the reduction of Co also allows for a cheaper, more environmentally friendly cathode. During electrochemical discharge, Li ions and their associated electrons are intercalated into the TMO2 host. There have recently been several investigations into how the intercalated lithium affects the electronic structure of the host.1-15 The results of these studies have led to recent debate over the amount of charge transferred to the O and TM ions. Analyses of the TM core edges, using X-ray absorption spectroscopy (XAS), magnetic susceptibility, electronic conductivity, and thermoelectric measurements, have shown that, upon lithium deintercalation, the TM ion is oxidized to the tetravalent state.1-3,9,10 Studies of the LiNi1-yCoyO2 system suggest that, during discharge, the Ni ions are immediately oxidized to the tetravalent state, whereas * To whom correspondence should be addressed. E-mail address:
[email protected].
the Co oxidation occurs only in the heavily delithiated state.1-3 In LiNi0.8Co0.2O2, the preferential oxidation of Ni over Co is attributed to the position of the Fermi level, which lies within the energy gap between the Ni2+/Ni3+ and Ni3+/Ni4+ redox couples. It is argued that the Fermi level drops into the Ni3+/ Ni4+ band during the early stages of delithiation and the nickel is oxidized. The t2g band of Co appears at a lower energy, requiring the removal of up to 80% of the Li atoms before the Fermi level is lowered sufficiently for Co oxidation.1 Although there is general agreement that the Co ion participates little in the redox processes that occur during cycling, the behavior of the Ni ion is still not well understood. The notion of a trivalent Ni ion in the fully lithiated material has been disputed by recent results of Montoro et al., which suggest that, in LiNi0.5Co0.5O2, the Ni ions are divalent and are partially oxidized to a trivalent state during delithiation.11 Absorption studies using hard X-rays have been used to investigate the electronic structure around the TM ions. However, these studies are unable to measure electronic transitions with low absorption energies, such as those contributing to the O K-edge. Therefore, these measurements are relatively insensitive to the atomic states of O. More indirect experiments, such as magnetic measurements, can be difficult to interpret and may be misleading because changes in the spin density, or the number of unpaired electrons around an ion, are not necessarily correlated with changes in the charge density. Soft X-ray absorption and transmission electron energy-losss spectroscopy (EELS) are well suited for studying the electronic structure around the O and TM ions at different states of lithiation. Recent studies of the O K-edge and Ni,Co L2,3-edges have shown that the O ion is responsible for accommodating much of the Li 2s electron during intercalation.12-15 These studies suggest an almost-constant net charge density about the TM ion, with significant changes in occupancy of the O 2p band. The details of the redox energies of the individual TM ions appear to be subordinate to changes occurring in the O valence. First-principles calculations by Wolverton and Zunger have shown that, in LiCoO2, the total charge density around the Co
10.1021/jp026484y CCC: $25.00 © 2003 American Chemical Society Published on Web 03/11/2003
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ion is unaffected by the lithium concentration. This behavior is attributed to a rehybridization of the Co-O bond, which produces invariant energy levels and electronic stability over a wide range of lithium concentration.16 Other first-principles calculations of LiCoO2 have suggested that much of the Li 2s electron is donated to the O, allowing the Co valence to remain predominately in the trivalent state.17-19 These ab initio studies add insight into the experimental results for the ordered LiCoO2 and LiNiO2 systems. However, calculations on a disordered distribution of Ni and Co atoms are not practical, because of the complexity of the unit cell of Li(Ni,Co)O2. Here, we report results from an experimental investigation using transmission EELS to elucidate changes in the electronic structure of Li1-xNi0.8Co0.2O2 with the amount of lithium removed, x. 2. Experimental Section Samples were prepared by chemically charging (delithiating) LiNi0.8Co0.2O2 powders, which were provided by Enax Inc. A previous investigation of the core edges of Li1-xCoO2 revealed similar results for chemically and electrochemically delithiated samples. However, chemically prepared samples are preferred, because spectral artifacts from binders and conductive diluents can be eliminated. In this study, an aqueous solution of potassium persulfate (K2S2O8) was used as an oxidant. The lithium concentration was altered by mixing powders of LiNi0.8Co0.2O2 into different amounts of the oxidizing solution, and samples were agitated for ∼24 h at 60 °C. The delithiated powders were filtered and rinsed several times in water to remove all the K2S2O8. To remove the residual water, the samples were dried in air at 60 °C for 24 h and stored in an argon atmosphere. The elemental analysis of the bulk material was performed using inductively coupled plasma-mass spectrometry (ICP-MS). Two methods were employed to determine the final lithium concentrations in Li1-xNi0.8Co0.2O2. First, the amount of lithium removed from the stoichiometric material, x, was measured in the K2S2O8 solution using ICP-MS. Second, the amount of lithium remaining in the delithiated oxides (1 x) was measured by ICP-MS. A structural analysis was performed using X-ray diffraction (XRD). Powder XRD patterns were acquired with an Inel model CPS-120 diffractometer, using Co KR radiation (λ ) 1.790 Å). The XRD results of the delithiated samples exhibited an (003) line width of ∆2θ ) 0.3°. These results were compared with those of Ronci et al., to ensure that the structure was not altered during delithiation and to confirm the ICP-MS measurements.20 Transmission electron microscopy and EELS measurements require an electron-transparent sample. To ensure that some of the particles in the Li1-xNi0.8Co0.2O2 were suitably thin, the powders were crushed with a mortar and pestle in Fluorinert and the fine particles were dispersed onto a holey carbon grid. The EELS spectra were acquired using a Gatan model 666 parallel-detection spectrometer in a Philips model EM 420 transmission electron microscope operated at 100 keV. The spectra were acquired with diffraction coupling (image mode), at a dispersion of 0.5 eV per channel. A large collection angle of ∼100 mrad was used to ensure spectral contributions from the entire Bethe ridge. The energy resolution was ∼1.5 eV. 3. Results The various core edges for the five Li1-xNi0.8Co0.2O2 samples are shown in Figures 1-3. Detector artifacts were removed by dividing out the instrument response. The full energy-loss spectra (0 e E e 950 eV) were then deconvolved using the Fourier-log method.21 (The deconvolution process removes the
Figure 1. (a) Li K-edge from Li1-xNi0.8Co0.2O2, labeled by x. The dashed line connects the Li K-edge peaks. (b) Normalized Li K-edge intensity vs measured lithium concentration (ICP-MS).
effects from plural scattering that distort the shape of the nearedge structure.) Finally, a background was fit to the pre-edge region, using a power-law decay function, and subtracted from beneath the core edges. The lowest-lying core edges for the Li1-xNi0.8Co0.2O2 system are shown in Figure 1a. The Li K-edge onset occurs at ∼58 eV, which is shifted by ∼3 eV with respect to lithium metal. The shift to higher energies in lithium-metal oxides is the result of the ionic bonding between the lithium and the host structure. The loss of the Li 2s electron reduces the screening of the nuclear charge, thereby increasing the binding energy of the core electron and increasing the ionization energy. This region of the energy-loss spectrum also contains contributions from the Co and Ni M2,3-edges at ∼60 and ∼68 eV, respectively. The Co and Ni M2,3-edges appear to be unaffected by the lithium concentration. The Ni and Li edges are adequately separated in energy, and although the Co and Li edges overlap, the contribution from Co is small enough so that changes at the Li K-edge can be resolved clearly. As expected, this figure shows a decrease in the intensity of the Li K-edge as lithium is extracted from the material. ICP-MS and XRD results from the bulk powders have provided averaged information on the chemical composition of the delithiated samples. However, energy-loss spectra are acquired from areas of the sample
